In general, when a ship is navigated in an unloaded or lightly loaded condition, the ship is charged with ballast water to ensure a predetermined draft so as to not only stabilize the hull but also prevent hull bottom slamming, propeller racing, and other undesirable phenomena. A ballast tank is typically charged with water at a cargo unloading point (cargo unloading place), and the water in the ballast tank is discharged at a cargo loading point (cargo loading place). Marine life at the cargo unloading point is transported along with the ballast water in the ballast tank to the cargo loading point and discharged into the waters at the cargo loading point. This results in change in the ecosystem, damage to the ecosystem, and other problems in the waters at the cargo loading point. Since ballast water is transported and discharged on a global scale, plankton and other marine life contained in ballast water are possibly transported to waters that are not their original habitats and seriously affect the ecosystems and industrial activities, such as fisheries, in those waters. The transportation of ballast water has therefore been taken into consideration as a global issue concerning marine environment protection and regarded as a serious problem particularly in recent years.
To solve such a problem, a variety of methods have been proposed, which includes a method for processing unnecessary ballast water in an on-land facility instead of discharging it into the sea, a method for sterilizing or purifying ballast water (e.g., JP-A-2004-284481, JP-A-2002-234487, and JP-A-2006-7184), and a method for forcibly performing offshore ballast water exchange with use of a pump or any other suitable circulation apparatus (e.g., JP-A-2002-331991 and JP-A-2001-206280).
When the method for processing unnecessary ballast water in an on-land facility is employed, however, an on-land facility for processing ballast water needs to be newly built. The method for sterilizing ballast water has not yet been put into practice because sterilization and purification have not been established as a technology for reliably trapping microorganisms. In the case of sterilization using chemicals, secondary contamination and other problems are also of concern. Therefore, on-land processing, sterilization, and purification of unnecessary ballast water still encounter difficult problems.
On the other hand, the ballast water exchange techniques for forcibly performing offshore ballast water exchange have been in actual use, which are known as a sequential method in which a ballast tank is completely emptied and then recharged with seawater; a flow-through method in which a ballast tank is charged with water and overflowed so that the ballast water is exchanged; and a dilution method in which a ballast tank is charged with water while the ballast water is discharged at the same time.
Any of the forced exchange methods as set forth above, however, requires installation of a seawater exchange system including a forced circulation apparatus and an inboard pipeline in the hull, and driving operation of the seawater exchange system to exchange seawater. At present, an achievable seawater exchange rate is approximately merely 83% even when the seawater exchange system introduces into the ballast tank, an amount of water that is three times as much as the capacity of the tank. In order to achieve a seawater exchange rate of 95% or higher, it is necessary to introduce into the ballast tank, an amount of seawater that is at least five times as much as the capacity of the tank. Therefore, if a sufficient seawater exchange rate is to be attained by a forced exchange type of ballast water exchanger, a large amount of fuel and power is consumed to drive a pump and other devices, and a large amount of time and manpower is needed for operation of the system.
An example of a ballast water exchanger which does not rely on a forced circulation apparatus or other powered apparatus is described, for example, in JP-A-11-29089 and JP-A-2005-536402, in which relatively high water pressure acting on a bow portion is used for intake of seawater.